Trilobita (trilobites; phylum Arthropoda) The most primitive arthropod class (or in some classifications a phylum, where the Arthropoda rank as a superphylum), known from more than 3900 fossil species. Inhabitants of Palaeozoic seas, the trilobites appeared first in the early Cambrian, had their widest distribution and greatest diversity in the Cambrian and Ordovician Periods, and became extinct in the Permian. The body was divided into three regions: an anterior cephalon, comprising at least five, fused segments; a mid-body (thorax), with a varying number of segments; and a hind region (pygidium). All three regions were divided by a pair of furrows running the length of the body, giving a trilobite appearance (i.e. a median or axial lobe, flanked on either side by a lateral lobe). The mouth was situated in the middle of the central surface of the cephalon. Paired gill-bearing limbs were attached to the membranous, pleural skeleton. X-ray studies show the eyes to have resembled the compound eyes of living arthropods. Trilobites ranged in size from 0.5 mm-long planktonic forms to those nearly 1 m in length; most species were 3–10 cm long.

trilobite (trī´ləbīt´), subphylum of the phylum Arthropoda that includes a large group of extinct marine animals that were abundant in the Paleozoic era. They represent more than half of the known fossils from the Cambrian period. The trilobite body was generally oval and flat and was divided into three roughly equal sections: the head, thorax, and tail. The name trilobite refers to a pair of furrows along the length of the animal that divided the body into three longitudinal regions. The body was covered by a mineralized shell. Because the dorsal, or upper, shell was thicker than the under shell, it has been the part best preserved in fossil form. Trilobites were abundant inhabitants of the Cambrian and Ordovician geological periods. They declined thereafter, possibly because they became food for cephalopods and later for fish, and became extinct in the Permian period. Trilobites are most closely related to the chelicerates, which include the horseshoe crabs and spiders.

See R. Fortey, Trilobite! Eyewitness to Evolution (2000).

trilobite An extinct marine arthropod belonging to the class Trilobita (some 4000 species), fossils of which are found in deposits dating from the Precambrian to the Permian period (590–280 million years ago). Trilobites were typically small (1–7 cm long); the oval flattened body comprised a head (covered by a semicircular dorsal shield) and a thorax and abdomen, which were protected by overlapping dorsal plates with a raised central part and flattened lateral portions, presenting a three-lobed appearance. The head bore a pair of antenna-like appendages and a pair of compound eyes; nearly all body segments bore a pair of Y-shaped (biramous) appendages – one branch for locomotion and the other fringed for respiratory exchange. Trilobites were bottom-dwelling scavengers.

trilobite Any of an extinct group of arthropods found as fossils in marine deposits, dating from the Cambrian to the Permian period. The body was oval, tapering towards the rear, and covered by a chitinous skeleton. Transverse divisions show segmentation, with each segment bearing a pair of jointed limbs. Most species were bottom-crawling, shallow-water forms, and ranged in size from 6mm (0.25in) to 75cm (30in).

tri·lo·bite / ˈtrīləˌbīt/ • n. an extinct marine arthropod (subphylum Trilobita) that occurred abundantly during the Paleozoic era, with a carapace over the forepart, and a segmented hindpart divided longitudinally into three lobes.

trilobite member of a group of extinct arthropodous animals having a three-lobed body. XIX. — modL. trilobītes; see TRI-, LOBE, -ITE.

trilobite A member of the fossil arthropod class Trilobita.

trilobite See TRILOBITA.